Magnetic devices, such as inductors and transformers, are employed in many different types of electrical devices including communications equipment and power supplies. In practice, most magnetic devices are fabricated of one or more windings, formed by an elongated electrical conductor, such as a wire of circular or rectangular cross-section, or a planar electrical conductor wound about or mounted to a bobbin composed of a dielectric material, such as plastic. In some instances, the electrical member is soldered to terminations on the bobbin. Alternatively, the electrical member may be threaded through the bobbin for connection directly to a metallized area of an underlying circuit board. A magnetic core may be disposed about the bobbin to impart a greater reactance to the magnetic device and thereby alter its operating characteristics.
As stated, transformers and inductors are very important components of switch-mode power supplies, i.e. power supplies that use switches to regulate an output voltage to account for variations in input and load conditions. Switch-mode power supplies, by nature of their operation and components can often generate a great deal of EMI. EMI can radiate from the power equipment and impair the function of other electrical equipment. Magnetic components, particularly transformers, are often major sources of EMI. Shielding such components can greatly reduce the EMI emitted by the component, however, such shielding must take safety regulations in mind.
A conductive strap can be wrapped around a power transformer to reduce the effects of stray magnetic fields around the core, which can cause both radiated and conducted EMI. For safety reasons the magnetic core is left floating, i.e. not at a defined potential, which allows equal spacing between the core and primary windings, and between the core and the secondary windings. The drawback to leaving the core floating is that the windings can capacitively couple to the core through the parasitic capacitance that exists between the windings and the core. The capacitive coupling can be further increased by the proximity of the conductive strap to the outermost windings. Additionally, if the power transformer is close to other components, the magnetic core may couple to them, creating common mode currents that can cause EMI.
Accordingly, what is needed is a shield for small magnetic structures that would keep the magnetic core at a fixed potential while electrically isolating it from the primary and secondary sides of the circuit.